...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE LOWER MISSISSIPPI VALLEY...
...SUMMARY...
Severe thunderstorms are likely from the Lower Mississippi Valley
toward the lower Ohio Valley Friday afternoon and evening.
Tornadoes, some strong, as well as damaging winds and hail are
expected.
...Synopsis...
A shortwave trough with positive tilt will move across the Southern
Plains on Friday, and into the middle MS/lower OH Valley by Saturday
morning. The primary zone of cooling aloft will remain roughly along
and north of a Texarkana to Memphis to Evansville line, but height
falls will occur even south of the jet core toward the northern Gulf
Coast. This wave will impinge upon a prominent upper high over the
Southeast, with a leading anticyclonically curved jet lifting north
across the OH River and toward the Upper Great Lakes.
The combination of differential divergence and low-level warm
advection looks to be maximized from the Ozarks into the lower OH
Valley from 21 to 06Z. Coincidentally, the surface low will deepen
most after 00Z as it moves from northern AR northeastward up the OH
Valley overnight. The low will move along an existing synoptic
front, with a northward-jumping warm front possible just ahead of
the low track from far southeast MO/southern IL into southern IN
after 00Z. A cold front will develop behind the low, with low-level
convergence maximized from northeast TX into western AR by 00Z,
pushing east across northern MS and western TN/KY after 06Z. More
subtle convergence is anticipated over southern portions of the warm
sector across the Sabine River and into LA.
The warm sector will be characterized by mid 60s F dewpoints from
the low track southward, with upper 60s to 70 F likely into northern
LA and central MS. Strong deep-layer shear will overspread the
region, with increasing low-level shear developing late in the
day/evening from the lower MS valley to the OH Valley as 850 winds
increase into the 50-70 kt range.
...Western and northern AR into the OH Valley...
Early day storms will be ongoing from eastern OK across northern AR
and toward the OH River along and north of a surface front in a zone
of glancing warm advection with a veered low-level jet. Some of this
activity could contain hail from OK into AR given elevated MUCAPE
over 1000 J/kg and long hodographs.
In the wake of the earlier activity, diurnal storms are expected to
form near the surface low and front extending south across western
AR by mid afternoon, with additional activity extending
northeastward along the developing warm front into southeast MO and
to the OH River. Supercells may mature over AR, producing tornadoes
especially later in the day toward 00Z as the low-level jet
strengthens rapidly. This threat will expand quickly northeastward
across the OH Valley, with a corridor of tornado or damaging winds
anticipated near the surface low track. Given the very strong shear
Friday evening, minimal uncapped surface-based instability will be
needed to produce significant severe storms. As additional model
guidance arrives and better observations exist into the Day 1
period, it is plausible that parts of the Moderate Risk could be
extended/shifted northward a bit across AR given the more favorable
large-scale lift over northern areas.
...Lower MS Valley...
The air mass will destabilize across this area well into the
evening, which will remain open warm sector with the cold front
stalling to the west. Severe weather is most likely late in the day
and evening as low-level moisture remains favorable, gradual cooling
aloft occurs, and the low-level shear increases substantially. The
fact that convergence will be subtle may be beneficial to the
supercell tornado threat, and deepening bands of convection may
eventually produce tornadoes from northern LA into central MS. Even
if storms in this area remain isolated, impressive hodographs with
0-1 SRH over 300 m2/s2 will favor supercells and tornadoes, perhaps
tracking for long distances given the lack of cold/stable air.
Therefore despite perhaps a weaker model signal over these southern
areas, the Moderate Risk has been maintained.
